Fix slow tests in CI

[teor2345]

Some tests take half an hour to run in CI, but under a minute to run in --release mode.

There might be some crates we can compile in release mode during tests, here's how to do that: https://github.com/ZcashFoundation/zebra/blob/8437fdbdfa1376982eeb0a8f2cdd9a93f227da86/Cargo.toml#L34-L39

If we had a flamegraph (#39), it would be easy to find those crates. But until we do, my guess would be:

• ark_ff
• ark_poly
• num_bigint Or their dependencies.

[teor2345]

Closed by #110, we just optimised the entire build.